Apparatus and method for measuring ground impedance of a circuit

Abstract

A ground continuity circuit is described. In one embodiment, a first voltage of a signal associated with an electrical line input to a circuit is measured with respect to a first resistance value of the circuit. A ground continuity test signal is asserted into the circuit that causes the resistance value of the circuit to change to a second resistance value. A second voltage of the signal is measured with respect to the second resistance value. A ground impedance value of the circuit is determined as a function of the first and second measured voltages and the first and second resistance values.

What is claimed is: 
     
       1. A method, comprising:
 measuring, with respect to a first resistance value of a circuit, a first voltage of a signal associated with an electrical line input to the circuit, wherein a first set of one or more resistors provides the first resistance value of the circuit; 
 asserting a ground continuity test signal into the circuit that causes a switch to close to change the resistance value of the circuit to a second resistance value by causing a second set of one or more resistors to be connected in parallel with the first set of resistors, wherein the second resistance value is lower than the first resistance value; 
 measuring, with respect to the second resistance value, a second voltage of the signal associated with the electrical line input; and 
 determining a ground impedance value of the circuit as a function of the first and second measured voltages and the first and second resistance values. 
 
     
     
       2. The method of  claim 1 , wherein the ground impedance value is determined by a ratio between the measured second voltage and the measured first voltage. 
     
     
       3. The method of  claim 2 , wherein the ratio being below a predetermined threshold indicates a high impedance of the grounding path. 
     
     
       4. The method of  claim 3 , further comprising:
 wherein the circuit is part of an electric vehicle charging system; and 
 preventing charging circuitry of the electric vehicle charging system from being energized when the ratio indicates high impedance of the grounding path. 
 
     
     
       5. The method of  claim 3 , further comprising:
 wherein the circuit is part of an electric vehicle charging system; and 
 interrupting charging circuitry of the electric vehicle charging system when the ratio indicates high impedance of the grounding path during charging operation. 
 
     
     
       6. An apparatus, comprising:
 a first set of one or more resistors providing resistance to a circuit; 
 a second set of one or more resistors connected in parallel with the first set of resistors by a switching mechanism, wherein the second set of resistors provides resistance to the circuit when the switching mechanism is closed and not when the switching mechanism is open; and 
 a processor operable to:
 periodically assert a ground continuity test signal into the circuit for operating the switching mechanism, and 
 determine a ground impedance value of the circuit based on a ratio between a first voltage measurement of the circuit when the switching mechanism is closed and a second voltage measurement of the circuit when the switching mechanism is open. 
 
 
     
     
       7. The apparatus of  claim 6 , wherein the ground impedance value is high impedance when the ratio is below a predetermined threshold. 
     
     
       8. The apparatus of  claim 7 , further comprising:
 charging circuitry connected to a load terminal for charging electric vehicles; 
 control circuitry for energizing and de-energizing the charging circuitry; 
 wherein the processor is further operable to prevent the charging circuitry from being energized when the ratio is below the predetermined threshold. 
 
     
     
       9. The apparatus of  claim 7 , further comprising:
 charging circuitry connected to a load terminal for charging electric vehicles; 
 control circuitry for energizing and de-energizing the charging circuitry; 
 wherein the processor is further operable to cause the control circuitry to de-energize the charging circuitry when the ratio is below the predetermined threshold. 
 
     
     
       10. The apparatus of  claim 6 , wherein the switching mechanism includes a transistor. 
     
     
       11. An apparatus, comprising:
 a means for measuring, with respect to a first resistance value of a circuit, a first voltage of a signal associated with an electrical line input to the circuit, wherein a first set of one or more resistors provides the first resistance value of the circuit; 
 a means for asserting a ground continuity test signal into the circuit that causes a switch to close to change the resistance value of the circuit to a second resistance value by causing a second set of one or more resistors to be connected in parallel with the first set of resistors, wherein the second resistance value is lower than the first resistance value; 
 a means for measuring, with respect to the second resistance value, a second voltage of the signal associated with the electrical line input; and 
 a means for determining a ground impedance value of the circuit as a function of the first and second measured voltages and the first and second resistance values. 
 
     
     
       12. The apparatus of  claim 11 , wherein the ground impedance value is determined by a ratio between the measured second peak voltage and the measured first peak voltage. 
     
     
       13. The apparatus of  claim 12 , wherein the ratio being below a predetermined threshold indicates a high impedance of the grounding path. 
     
     
       14. The apparatus of  claim 13 , further comprising:
 wherein the circuit is part of an electric vehicle charging system; and 
 a means for preventing charging circuitry of the electric vehicle charging system from being energized when the ratio indicates high impedance of the grounding path. 
 
     
     
       15. The apparatus of  claim 13 , further comprising:
 wherein the circuit is part of an electric vehicle charging system; and 
 a means for interrupting charging circuitry of the electric vehicle charging system when the ratio indicates high impedance of the grounding path during charging operation.